Piezoelectric Transducers
Piezoelectric ceramics are currently the material of choice for ultrasonic transducer applications offering relatively high coupling (kij), a wide range of dielectric constants (K), and low dielectric loss. These merits translate into transducer performance in the form of relatively high sensitivity, broad bandwidth, impedance matching and minimal thermal heating.
Pb(Zr.sub.l-x,Ti.sub.x)O.sub.3 (PZT) ceramics have been a mainstay for high performance transducer applications. Compositionally, PZT ceramics lie near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases. MPB compositions have anomalously high dielectric and piezoelectric properties as a result of enhanced polarizability arising from coupling between two equivalent energy states, i.e. the tetragonal and rhombohedral phases, allowing optimum domain reorientation during the poling process. Further modifications, using acceptor and donor dopants, give a wide range of piezoelectric compositions.
Alternative MPB systems can be found in relaxor-based ferroelectrics and their solid solutions with PbTiO.sub.3 (PT). Lead based relaxor materials are complex perovskites with the general formula Pb(B.sub.1 B.sub.2)O.sub.3, (B.sub.1 =Mg.sup.2+, Zn.sup.2+, Ni.sup.2+, Sc.sup.3+ . . . , B.sub.2 =Nb.sup.5+, Ta.sup.5+, W.sup.6+. . . ). Characteristic of relaxors is a broad and frequency dispersive dielectric maxima. Some relaxor-PT compositions such as modified Pb(Sc.sub.1/2 Nb.sub.1/2)O.sub.3 -PbTiO.sub.3 (PSN-PT) seem to possess superior dielectric and piezoelectric properties compared to that of PZT ceramics. However, if analyzed with respect to the ferroelectric transition temperature Tc (the temperature at which the material transforms from the prototypical non-ferroelectric to ferroelectric phase being associated with a spontaneous polarization and large dielectric anomaly), no one type of ceramic offers significant advantages in overall transducer performance.
Enhanced piezoelectric activity of MPB-based ceramics by compositionally adjusting the Curie temperature (Tc) downward relative to room temperature, comes with the expense of more temperature dependent properties and less polarization stability, i.e. aging and loss of piezoelectric activity.
Though relaxor-PT ceramics do not offer enhanced dielectric and piezoelectric properties comparable to PZT ceramics of similar T.sub.c 's, it is the single crystal form of relaxor-PT ceramics that exhibits ultrahigh piezoelectric properties not currently available with piezoelectric MPB ceramics. This key distinction was first realized by Yonezawa et al. (Journal of Japanese Society of Powder Metallurgy, Vol 16, pp 253-258, 1969) and later by Kuwata et al. (Ferroelectrics, Vol 37, pp 579-582, 1981) for MPB compositions of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 (PZN-PT) systems with k.sub.33 values ranging 80 to 90%, followed by crystal growth and evaluation of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 (PMN-PT).
Although high coupling and piezoelectric properties of the PZN-PT system, first reported in 1969 and later in the PMN-PT system (1989), have been known for several years, their potential for high performance biomedical ultrasound transducers and related devices has only been recognized recently. Serious efforts on the development of Pb(B.sub.1 B.sub.2)O.sub.3 -PT crystals for high performance transducers is disclosed in U.S. Pat. Nos. 5,402,791 and 5,295,487 to Saitoh et al. Therein are disclosed relaxor based ferroelectric single crystals of lead zinc niobate--lead titanate (Pb(Zn.sub.1/3 Nb.sub.2/3).sub.l-x Ti.sub.x O.sub.3) and lead magnesium niobate--lead titanate (Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-x Ti.sub.y O.sub.3 ) solid solutions, where 0.05&lt;x&lt;0.2 and 0.05&lt;y&lt;0.2, respectively. It should be noted that the range 0.05&lt;y&lt;0.2 for Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-y Ti.sub.y O.sub.3 solid solutions described by Saitoh et al. includes the non piezoelectric relaxor (microdomain) region (0.05&lt;y&lt;0.15) at room temperature. For 0.15&lt;y&lt;0.20, according to this invention, crystals of Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-y Ti.sub.y O.sub.3 solid solutions exhibit longitudinal coupling&lt;80%, only comparable to MPB ceramics.
When optimum crystallographic cuts, in this case pseudocubic &lt;001&gt;, are utilized for rhombohedral crystals, an electric field induced phase transformation from rhombohedral to tetragonal phase can occur under dc bias, because the polar axis in the rhombohedral phase is &lt;111&gt;. The phase transition field is strongly dependent upon composition, i.e., the closer to MPB, the lower the transition electric field level because two crystallographic states (rhombohedral and tetragonal) energetically become closer to each other and fmally coexist at room temperature when the composition lies on a MPB (0.09&lt;x&lt;1.0 and 0.35&lt;y&lt;0.40, for Pb(Zn.sub.1/3 Nb.sub.2/3).sub.l-x Ti.sub.x O.sub.3 and Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-y Ti.sub.y O.sub.3, respectively).
The rhombohedral-tetragonal phase transition results in discontinuous changes in piezoelectric properties and dimensions. This dimensional change can affect transducer performance during fabrication processes, such as mechanical clamping during poling. Transducer poling normally includes field cooling (cooling under dc-bias from the temperatures above Tc).
FIG. 3 shows a plot of dielectric constant vs. temperature for Pb(Zn.sub.1/3 Nb.sub.2/3).sub.0.92 Ti.sub.0.08 O.sub.3, exhibiting transitions from rhombohedral to tetragonal (100.degree. C.) and from tetragonal to cubic (160.degree. C.). Apart from composition, the phase transition field decreases with increasing temperature. Therefore, increasing temperature under dc bias causes the rhombohedral phase to readily transform into the tetragonal phase.
It should be noted that transducers are attached to multiple quarter wave length matching layers and/or take the form of a low impedance composite comprised of piezoelectric elements with intermediate passive polymer layers, in order to couple acoustic energy and decouple lateral modes. Therefore, successive cooling under bias will cause the crystals to transform back to a rhombohedral phase after the passive polymer becomes significantly rigid at low temperature, resulting in elastically clamped crystals, leading to internal stresses and mechanical failure. Also, stability of rhombohedral phase becomes an issue as the transducer is driven at increased frequencies. To increase driving frequency and thereby obtain enhanced sensitivity, the transducer must be thinner, resulting in an increased electric field at a same driving voltage. Therefore, to avoid a phase transformation with a &lt;001&gt; rhombohedral crystal, the driving voltage must be limited. To apply higher voltages for pulse generation, the composition of a rhombohedral crystal should be located appropriately away from the MPB.
In summary, the problems exhibited by compositions of relaxor based single crystals are:
1) Lead magnesium niobate--lead titanate (Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-y Ti.sub.y O.sub.3) is not piezoelectric at room temperature when 0.05&lt;y&lt;0.15 and does not exhibit piezoelectric properties superior to conventional MPB ceramics when 0.15&lt;y&lt;0.20. PA1 2) For compositions close to MPB for rhombohedral lead zinc niobate--lead titanate (Pb(Zn.sub.1/3 Nb.sub.2/3).sub.l-x Ti.sub.x O.sub.3), and lead magnesium niobate--lead titanate (Pb(Mg.sub.1/3 Nb.sub.2/3).sub.l-x Ti.sub.y O.sub.3), where 0.05&lt;x&lt;0.09 and 0.33&lt;y&lt;0.40, the rhombohedral-tetragonal phase transformation occurs at relatively low fields. PA1 1) Low total strain (&lt;0.15%). PA1 2) Low strain energy density due to low piezoelectric coefficient (d.sub.33) &lt;700 pC/N and electrical breakdown strength. PA1 3) Significant hysteresis which leads to substantial heat generation and poor positioning accuracy.